Locking mechanisms for oilfield equipment often include a device called a hang-off dog. The dog is put into a locking position between two components, such as a tension ring and a diverter housing, to hold a position or take a load. For example, the hang-off dog can be part of a tension ring that can maintain a riser in stable tension when offshore platforms heave. A telescoping joint or slip joint is typically connected to the riser to allow compensating for such heaves. The telescoping joint is also typically locked to the tension ring via dogs or pins on the tension ring that mate with a groove on the telescoping join. When the riser is retrieved during routine operation or emergency situations, the hang-off dog can be actuated to engage a diverter support housing connected to the rig, thus preventing the riser from falling and injuring personnel. The hang-off dogs can be manually or hydraulically actuated to engage the diverter support housing.
To prevent the accidental unlocking of the hang-off dogs, a manual lockout or positive stop can be employed to secure the hang-off dog in a locked position. When the lockout is engaged, the hang-off dog is locked in an engaged position. When the lockout is disengaged, the hang-off dog is free to move between an engaged and a disengaged position. One method of providing a positive stop for hang-off dogs includes using a metal gate that requires a person to physically flip the gate over behind the hang-off dogs. This requires sufficient clearance above the tension ring to allow the gate to flip over. When the gate is flipped back over in the opposite direction to allow the hang off dogs to retract, the gate rests on top of tension ring which could then prevent movement of tension ring during normal operation. Further, this method requires that each hang off dog have an individual, hydraulically actuated gate. The method is costly due to the individual gates and the extra machining to provide flats on which to mount the hydraulic gates.
A need exists for a technique to provide a cost-effective, positive stop for the hang-off dogs on the tension ring to prevent the accidental retraction of hang-off dogs. Further, the positive stop technique should require minimal space in which to operate. The following technique may solve these problems.